Homogeneous hydrogenation catalysts for the production of hydrogenation reactants are well known in the art, with many systems being based on rhodium metal combined with phosphine ligands. Examples of such catalysts were first described in J. A. Osborn, F. H. Jardine, J. F. Young and G. Wilkinson, J. Chem. Soc. (A) (1966) 1711.
The Osborn et al. paper describes tile hydrogenation of hydrogenatable products using a catalyst precursor of the formula [RhCl(PPh.sub.3).sub.3 ] and a pressure of hydrogen gas of one atmosphere, this system presently remains the catalyst of choice for many homogeneous hydrogenation applications even though it has not ken optimized or improved despite considerable efforts to do so. One disadvantage of this system is that it has low selectivity in the hydrogenation of different hydrogenatable sites within the same reactant.
In an effort to improve on this system work described in R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc., 98 (1976) 2143 employed catalyst precursors of the general formula [Rh(diene)L.sub.n ].sup.+ X.sup.- ( where "client"is a hydrocarbon diene such as cyclooctadiene or norbornadiene, L=tertiary phosphine, n=2, X.sup.- =ClO.sub.4.sup.-, BF.sub.4.sup.-, PF.sub.6.sup.-. Only a moderate improvement in reactivity ascribed to the use of coordinating solvents was demonstrated in this work. Also of note was that the more basic tertiary phosphine ligands, e.g., PPhMe.sub.2 promoted the hydrogenation rates but also promoted the isomerization of the reactants. For example, 1-hexene is readily isomerized to cis- and trans-2-hexene under the, conditions employed.
The use of this catalytic system utilizing non-coordinating solvents is described in R. H. Crabtree, A. Gautier, G. Biordano and T. Khan J. Organomet. Chem. 141 (1977) 113 where catalyst precursors of the general formula [Rh(cod)L.sub.2 ].sup.+PF.sub.6.sup.- (cod=Cyclooctadiene, L=tertiary phosphine or amine) in dichloromethane solvent gave significant improvement in catalytic activity over the results reported by Schrock and Osborn op. cit. In the work of Crabtree et al. the highest reactivity was obtained using the complex [Rh(cod)(PPh.sub.3).sub.2 ].sup.+PF.sub.6.sup.- in dichloromethane solvent, particularly for the hydrogenation of 1-hexene to hexane.
Following this work-further reactivity improvements were reported in J. Halpern and C. R. Landis, J. Organomet. Chem., 250 (1983) 485 and J. Halpern, Phosphorus and Sulphur, 18 (1983) 307 wherein complexes of the general formula [Rh(diolefin)P.sub.2 ].sup.+ X.sup.- were employed as catalyst precursors (P.sub.2 =a chelating bis-tertiary phosphine ligand, X.sup.- =a non-coordinating anion, typically BF.sub.4.sup.-) . It was found that the use of chelating bis-phosphine ligands of the type Ph.sub.2 P(CH.sub.2).sub.n PPh.sub.2 (where n=2, 3, 4, 5) gave significant enhancement of catalytic activity particularly when n=3 or 4.
Therefore it is noted that the reactivity of rhodium-phosphine catalysts in homogeneous hydrogenation can depend strongly on the solvent choice and on the nature of the phosphine ligand.
Metals other than rhodium have been employed in catalytic homogeneous hydrogenation utilizing phosphine complexes of those metals.
An example of such a catalyst is given in G. Wilkinson, P. S. Hallman and B. R. McGarvey J. Chem. Soc. (A) (1968) 3143 where a compound of the formula [RuHCl(PPh.sub.3).sub.3 ] was shown to be an active catalyst for the hydrogenation of terminal alkenes such as 1-hexene but was a poor catalyst for hydrogenation of internal or cyclic alkenes such as cyclohexene. Similar such compounds containing ruthenium or osmium have been reported in U.S. Pat. No. 3,454,644.
Another example of such catalysts is the use of iridium in R. H. Crabtree, H. Felkin and G. E. Morris J. Organomet. Chem., 141 (1977) 205 and R. H. Crabtree, Acc. Chem. Res. 12 (1979) 331 where compounds of the general formula [Ir(cod)L.sub.2 ].sup.+ PF.sub.6.sup.- (L=tertiary phosphine or amine) in dichloromethane solvent where found to be highly active hydrogenation catalyst for all types of alkene reactants.
A review covering applications of some of the previously mentioned catalyst precursors deemed to be the state of the art can be found in J. M. Brown, Angew. Chem. Int. Ed. Engl. 26 (1987) 190 which include particularly the catalyst precursors having the formulae [RhCl(PPh.sub.3).sub.3 ], [Rh(nbd)dppb].sup.+ BF.sub.4.sup.- and [Ir(cod)PCy.sub.3 (Py)].sup.+ PF.sub.6.sup.- (nbd=2,5 norbornadiene, dppb=1,4 bisdiphenylphospinobutane, cod=1,5 cyclooctadiene Cy=cyclohexyl and Py=pyridine).
Also of note is the use of chiral bis tertiary diphosphines in asymmetric hydrogenation with rhodium(I) catalyst precursors. Coverage of this application in both industrial and laboratory processes is given along with many related references in W. S. Knowles, Acc. Chem. Res. 16 (1983) 106 and H. B. Kagan, Bull. Soc. Chim., 5 (1988) 846. Interest in this area of hydrogenation has been intense and there are a number of patents related to synthesis and application of several rhodium-chiral diphosphine catalyst precursors: U.S. Pat Nos. 3,419,907; 3,849,490; 3,878,101; 4,166,824; 4,119,652; 4,397,787 and U.S. Pat. No. 4,440,936.
A number of rhodium-phosphine and rhodium-phosphite catalyst precursors have appeared in patent literature many of which are designed only to carry out specific hydrogenations of a specific reactant: U.S. Pat. Nos. 4,999,43, 4,911,865, 4,857,235, 4,863,639 and U.S. Pat. No. 4,743,699.